Dictionary:

perfidious

  (pər-fĭd'ē-əs)

Of, relating to, or marked by perfidy; treacherous. See synonyms at faithless.

adjective

Not true to duty or obligation: disloyal, faithless, false, false-hearted, recreant, traitorous, treacherous, unfaithful, untrue. See continue/stop/pause, trust/distrust.

Definition: treacherous
Antonyms: faithful, loyal

The adjective has one meaning:

Meaning #1: tending to betray; especially having a treacherous character as attributed to the Carthaginians by the Romans
  Synonyms: punic, treacherous